The present invention relates in general to protective containers or packaging for handling, shipping, and/or storing rigid generally planiform devices subject to damage by static electricity discharges, and more particularly to static electricity discharge protective containers or packaging for printed circuit boards, solid state semiconductor devices and chips of generally panel or planiform shaped configuration and similar devices subject to possible damage through discharge of static electricity, having means for guiding and positioning the printed circuit boards or like electrostatic discharge susceptible (ESDS) devices in like oriented parallel vertical planes or similar predetermined orientation.
Static electricity and protection against electrostatic discharges (ESD) is becoming more and more recognized as a significant problem in the handling, shipping, and storing of ESDS electronic parts or printed circuit boards which contain ESDS electronic parts; such electronic parts, for example, as any electronic part incorporating: accessible metal oxide semiconductor structures, small cross-sectional area semiconductor junctions, low power film resistors, long thin metalization strips, piezoelectric crystals, and closely spaced electrode structures, hereinafter frequently referred to as ESDS devices. Packaging of such devices for shipping, transferring from one location to another, and storing at production and service lines present significant problems in protecting such devices against static electricity discharges which might cause damage to the devices or to circuit components incorporated in them.
Previous to May, 1980 industry had no standard yardstick to measure device susceptibility levels. Since that time, military standard DOD-STD-1686 has provided such a yardstick in establishing classification levels of susceptibility. Three classes have been identified: class 1 includes those devices most susceptible and which fail when subjected to 1000 v from a human body equivalent circuit of 100 pf and 1500 ohms; class 2 includes devices which are not damaged by 1000 v but are damaged by 4000 v from the same human body equivalent circuit mentioned above sufficiently to be out of specification tolerance; class 3 includes devices which are not damaged by 4000 v but are damaged by 15,000 v sufficiently to be out of specification tolerance. With this classification system and the growing awareness of ESD, some protective packaging has been found inadequate to protect the most susceptible devices in the class 1 susceptibility classification.
A number of approaches have been tried to provide the desired protection of such devices against electrostatic discharge (ESD) events. In the early 1970's the electrically conductive black carbon-impregnated polyolefin plastic bags, and the high surface resistance, but groundaable pink, antistatic chemical-impregnated polyethylene bags were practically the only ESD protective packaging in use. When new multi-laminate metal coated bags became available, data was forthcoming showing how both the pink antistatic polyethylene bag and the conductive black polyolefin bags did not prevent the most susceptible parts such as the 3N157 from being damaged. The reason for this is their surface resistance is so high that potential difference develops across the bag which will cause an electric field to exist in the bag long enough to cause failure. Thus, for ESD protective bags to provide adequate shielding, sufficiently low surface resistance is necessary to protect the most susceptible class 1 devices.
What has not become widely apparent to the the related trade is that ESD protective packaging just being electrically conductive is not enough. For example, damaging conditions can occur in an arrangement such as disclosed in Cohen U.S. Pat. No. 4,308,953 involving a box-like container having electrically non-conductive cardboard walls provided with electrically conductive material (coated) on the opposite sides thereof and electrically conductive holding panels defining slots or grooves for guiding and locating the edges of the planiform printed circuit boards or ESD sensitive devices. As a person is charged walking on an electrostatic generating (ESG) surface, such as a nylon carpet, holding such a conductive container the container will build up a charge which will lie on the outside surface of the container, thus storing electrostatic energy on the container. If one side of this charged-up container is touched against a grounded metal surface, the side touching the metal surface will go instantly to ground but the opposite side will still be at the previous potential. If this surface conductive container is retaining rigidly between the two sides ESDS printed circuit cards with class 1 devices such that a moderately low resistance path exists from the edges of the printed circuit board to the exterior surfaces and if the outer surface conductivity is not high enough, a transient electric field will pass through the printed circuit board, which can damage the class 1 ESDS devices on the board, especially those that are voltage susceptible as opposed to those that are energy susceptible.
Another proposed solution was to provide a container in generally box form made of conductive plastic material so that the entire container was conductive. Conventionally, such a container was molded entirely from plastic material and provided with formations to receive the static sensitive devices. Such molded plastic containers were expensive to manufacture and also did not eliminate the possibility of the devices picking up static charges.
Other proposed arrangements have included cardboard containers provided on the interior walls with anti-static plastic members, wherein the cardboard container was not electrically conductive, and the anti-static plastic members in direct contact with the ESD susceptible devices, which also were not always electrically conductive, failed to provide the desired protection against unwanted static buildup. Still another arrangement involved cardboard shipping containers or boxes provided with an electrically conductive coating sprayed on the interior of the box, some of which additionally included multiple partitioning arrangements for receiving the ESD devices, but these did not ensure suitable protection of the devices and charge bleed off when placed on a grounded surface. Examples of these prior art approaches are found in Ohlbach U.S. Pat. Nos. 4,160,503; 4,211,324; and 4,293,070.
Study has revealed that the requirements the ESD protective containers must satisfy is to contain the ESD susceptible device or printed circuit board in such a way to permit the exposure of such devices to high fields and induced charge separation when they are out of the ESD protected work area where they are packaged or unpackaged, without any possible electrostatic discharge harm to the ESD susceptible devices inside the container, and to also enable the container to be brought into an ESD protected work area by having all parts of the container groundable so that no surfaces will retain charge which could induce charge on objects or devices in the container which ultimately would lead to an ESD event.
The containers disclosed in the above-identified Ohlbach patents do not achieve these design criteria, because the exterior surfaces will not sufficiently bleed-off a charge accumulated on the conductive interior surface as may occur over a period of time in a charge environment. When the properly grounded operator handles the container by touching only the non-conductive exterior, the charge on the conductive surface is deliverable with a high probability of being a damaging ESD event if the ESDS device or circuit board is touched before interior surfaces. There is even a possibility of a damaging electrostatic discharge from the charge redistribution process even if the interior conductive surface of the container is touched first by the grounded operator.
An object of the present invention, therefore, is the provision of a novel box-like package or container for electrostatic discharge susceptible devices such as contained on printed circuit boards or the like, wherein the container is formed of electrically surface and volume conductive paperboard or chipboard forming a rigid box, overwrapped with electrically surface and volume conductive paper, and provided with a thermoformed plastic insert filler formed of anti-static plastic material providing parallel ribs arranged in vertical planes spaced equidistant from each other defining grooves or slots for receiving and positioning the edges of the printed circuit boards or planiform ESD susceptible devices to be packaged therein, providing improved protection for the ESD susceptible devices to be packaged against undesired damaging static discharge events.
Another object of the present invention is the provision of an electrostatic discharge protective container or package as described in the immediately preceding paragraph, which incorporates sufficient resistance properties at all points along the exterior surfaces of the container due to the beneficial effect of parallel resistance of the conductive paper and conductive paperboard to the interior ESD susceptible devices so that during charge redistribution when a highly charged object approaches near the container, the path of least resistance is around the walls of the container and not through the ESD susceptible devices packaged therein due to the volume resistance of the conductive paperboard which dissipates significant potential differences and the anti-static insert filler which will also dissipate potential differences, and wherein all parts of the container are groundable such that charge bleed-off is ensured when the container rests on a properly grounded surface.
Other objects, advantages, and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction wiht the accompanying drawings illustrating a preferred embodiment of the invention.